1. Field of Invention
The present invention relates to an apparatus and a method for controlling a display backlight, and more particularly to an apparatus and a method for adjusting backlight intensity according to image data of a transmittive display.
2. Description of the Related Art
In the total power consumption of a display, the backlight thereof takes the most share. In terms of a conventional transmittive display (an LCD or an LCD projector, for example), the backlight intensity thereof is usually unchanged regardless of a bright image frame or a darker image frame the display gives, wherein an optical gate (the liquid crystal cell) is adjusted to decay the excessive light or to allow more portion of light passing through according to the different brightness or color of a specific pixel. In order to save power or increase contrast, an idea by adjusting the backlight intensity to achieve these goals in response to different color or brightness of a frame was emerged. In the following paragraphs, a transmittive LCD is described as example.
In the total power consumption of an LCD, the backlight thereof takes the most share, which is particularly impressive in the applications of portable products, such as a notebook PC or a personal digital assistant (PDA) where the backlight consumes up to 20˜30% of the entire power. With a transmittive LCD, a user can adjust backlight intensity thereof to fit a user's preference and ambient brightness; however, the backlight intensity, i.e. the power consumption, is the same, even for a darker frame. For a pixel thereof to get darker, a driving circuit is used to rotate the liquid crystal molecules of the LCD for decaying the excessive light. On the other hand, an alternative measure for saving power is that for the darker displayed frame, the backlight intensity is reduced in associate with increasing color values of the display pixels accordingly (i.e. to allow more portion of light to pass through). With such a novel method, the power is reduced while maintaining the original luminance and contrast. Furthermore, this method is easy to extended to include contrast enhancement and color adjustment capabilities.
To offer the power-saving capability, an apparatus for dynamically adjusting backlight of an LCD panel was provided. FIG. 1 is a block diagram showing a conventional apparatus for dynamically adjusting backlight of a LCD panel. Referring to FIG. 1, the apparatus comprises an image analyzer 100, an image compensator 102, a backlight controller 104 and a pulse width adjustor (PW adjustor) 106. By using the apparatus, all pixel signals of a frame are input to the image analyzer 100 to find out the maximum pixel color value of the frame. As an example, it is assumed that the maximum color value of all the pixels is [200] and the upper limit of the color value is [255] corresponding to an 8-bit display. The image compensator 102 respectively multiplies all pixel color values C by a factor S to obtain updated pixel color values C′, C′=C*S, wherein the factor S is the quotient of the upper limit of the color values by the maximum color value of all the pixels in a frame; thus, S=255/200 herein. In addition, the backlight controller 104 divides the original backlight luminance L by the factor S (i.e. 255/200 herein) to obtain an updated backlight luminance L′ (L′=L/S), followed by sending L′ to the PW adjustor 106 for obtaining a reduced backlight luminance of L′=L/S.
By using the mentioned scheme, all color values of all pixels are magnified by a factor and the backlight is adjusted to be darker by the same factor. If the relationship of color value vs. brightness and the relationship of backlight vs. intensity are ideally proportional (i.e. Y=CX), the scheme would be applicable to achieve power-saving without affecting the display quality. In fact, however, the relationship between color values of RGB pixels (red, green and blue pixels) and brightness and the relationship between backlight and intensity are not proportional. That is, the corresponding to a color value [100] is not double of the brightness corresponding to a color value [50]. People who skilled in the art knows, the relationship between brightness and color value is a so-called Gamma function of a display, which is a nonlinear function. Therefore, during an adjustment process, the brightness and the color of a frame would be dramatically changing resulting in undesired and poor image quality, and such degradation is unacceptable by users.